1. Field of the Invention
This invention concerns a method for the production of highly pure silicon or other semiconductor material, and is more particularly concerned with a method of producing highly pure semiconductor material by depositing the semiconductor material from a corresponding reaction gas at the outer surface of rod shaped carrier members which are heated to the depositing temperature in the reaction gas. The members consist of the same semiconductor material and are connected in series with respect to an operational current source which provides a direct current for heating the carrier members to the depositing temperature, and wherein prior to depositing the rod shaped carrier members are preheated by a multiphase voltage source with a high voltage.
2. Description of the Prior Art
A prior method of providing highly pure semiconductor material is disclosed in the French Pat. No. 1,125,207 and in the German published application No. 1,212,948. The preheating of the rodshaped carrier members, in particular consisting of highly pure silicon, by means of applying a sufficiently high auxiliary voltage has the advantage of all known methods of decreasing the resistance of the carrier member from a high value at room temperature to such a low value that the operational current source, operating at a considerably lower voltage, can further heat the carrier rods. Naturally, the goal of heating is achieved more quickly if the available auxiliary voltage is higher. Based on the auxiliary voltage, an electrical current of at first low strength flows in the carrier members which, however, develops sufficient heat in order to not only cover heat losses at the surroundings but also to gradually increase the temperature of the rods, and therefore is capable of decreasing rod resistance. Therefore, if a sufficiently high auxiliary voltage is elected, a static condition will not occur, but a successive increase in temperature and current will be experienced, and therefore a constant decrease of the electrical resistance of the carrier will be achieved, due to its decreasing temperature-resistance characteristic, until the operational current source, which provides a substantially lower voltage, is capable of supplying a current which can further heat the carrier rod notwithstanding this low voltage. If necessary, the preheating or "igniting" of the carrier rods can be supported by means of infra-red irradiation of the carrier rods.
On the one hand, preheating of the carrier rods requires a rather high voltage (several kV/m of the carrier rod). On the other hand, the load current during preheating is only in the order of magnitude of a millampere. A lower voltage is required for the operational source, however, several thousand ampere of heating current is experienced. This means, that for the operational source, a low voltage transformer and for the auxiliary voltage source a high voltage transformer are required.
It is possible, without particular difficulty, to heat several meters of carrier rods to the depositing temperature by means of an operational current source supplied by a current supply network in the case where silicon is used as a semiconductor. This fact is utilized in that, instead of a correspondingly long carrier rod, several carrier rods with shorter lengths are used, which rods are preferably arranged in the same reaction chamber and are connected in series with respect to the current source. However, a similar procedure with respect to the auxiliary voltage source which serves for preheating results in very high voltages. In order to avoid these, according to German Pat. No. 1,212,948, the carrier rods can be preheated individually by an auxiliary voltage source at correspondingly lower voltages in order to apply the rods individually to the operational current as soon as they are sufficiently preheated until such time as the operational current source is completely loaded. Such measures are, however, very time consuming.